Field of the Invention
Stiebel, U.S. Pat. No. 4,419,558 (Dec. 6, 1983) and A. Stiebel, C. Ulmer, D. Kodrack, B. Holmes, "Monitoring and Control of Spot Weld Operations," SAE Technical Paper Series, No. 860579 (1986), Issue No. 148-7191 describe monitoring and controlling electrical resistance spot-welding by measuring displacements of the electrodes during welding. After the squeezing force is applied by the electrodes to the work pieces and the supply of welding current is initiated, the metal at the work site first expands thermally as it heats (expansion) and then flows plastically as it softens and fuses (indentation). The electrodes are displaced by the expansion and indentation of the metal at the weld site as well as by the expansion and contraction of the electrodes. Thus, measurements of the displacement of the electrodes during formation of the weld contain information indicative of the state of the metal at the weld site. It has long been known that moderate indentation almost always ensures a good weld. The ability to measure the onset of indentation makes it possible, therefore, to shut off the welding current upon detection of indentation with a high level of assurance that a good weld has been formed. The Stiebel patent and the Stiebel et al technical paper referred to above are incorporated by the foregoing reference to them into the present specification.
In the method and apparatus of the Stiebel patent (and the Stiebel et al. technical article) consistent measurements of displacement are assured by interposing a mechanical compression spring between the piston of an air cylinder (or its equivalent) that moves the movable electrode into engagement with the work piece and the movable electrode. A load cell associated with the spring detects the changes in the load imposed on the spring as the movable electrode is displaced upon expansion and indentation of the metal of the work pieces at the weld site during formation of the weld. Compressing the spring during expansion provides changes in the resulting forces in the spring and thus on the load cells that are directly proportional to the displacement of the movable electrode. Without the spring, for example with a hydraulic or pneumatic cylinder directly working on the movable electrode, the piston is theoretically free to displace with the movable electrode in direct correspondence with the electrode movements, this providing no change in load and no opportunity to detect electrode displacements by detection of load changes. In practice, friction, both static or dynamic, affect the load, and strain, on the electrode and preclude consistent monitoring of electrode displacement by detecting load changes. Moreover, in many spot-welding environments, especially in car and truck manufacturing, several welding guns are working on the work piece simultaneously and vibrations of the work piece caused by other guns effect the signals picked up by the load detector of any particular gun.
The apparatus and method of the Stiebel patent were demonstrated by thorough testing to provide reliable and consistent load signals for use in controlling the welding process. An important practical disadvantage of the Stiebel apparatus, however, is that it requires specially built welding guns--guns produced with a fixed stop, a load cell, and a mechanical compression spring. It would require huge expenditures to replace the tens of thousands of welding guns currently in use with the special guns of the Stiebel et al patent.